In general, it is desirous to utilize filters in display technology, diagnostic systems, optical equipment, and other lighting systems to attenuate or accentuate particular types of electromagnetic radiation. For example, certain displays and visual equipment may accentuate particular colors in the visible light spectrum and attenuate other colors in the non-visible and the visible light spectrum. Accordingly, these displays can utilize filters to provide accentuation and attenuation for certain wavelengths of light. In another example, certain diagnostic equipment, such as x-ray equipment, may require that certain wavelengths be filtered so that film and detectors are not improperly exposed to electromagnetic radiation. Filters can be used to protect components from certain wavelengths of electromagnetic radiation.
In one particular filter application, displays and other equipment utilized in military, sports, and transportation activities are often employed in tandem with night vision equipment. These displays and equipment conventionally utilize a filter to accommodate the night vision equipment. Issues related to the use of displays and night vision equipment are described below with reference to an aviation application, although the below-mentioned issues are relevant to any applications of displays, night vision equipment, optical systems, diagnostic equipment, or lighting systems requiring attenuation or accentuation of certain wavelengths of electromagnetic radiation.
Certain aviation displays are color displays that are utilized with night vision imaging systems (NVIS). These displays provide visual information to captains, pilots, drivers and operators of ships, aircraft, and vehicles. The viewer of the color display often wears NVIS goggles at the same time he or she observes information from the color display.
Conventional NVIS goggles are sensitive to light in the infrared, near infrared, and visible red spectrum (wavelengths of light). NVIS goggles are typically sensitive to light between 425 nm and 1000 nm wavelengths. At 600 nm, the sensitivity rapidly increases and reaches a peak at 760 nm. The near infrared sensitivity of NVIS goggles allow the pilot or person wearing the goggles to see objects which cannot ordinarily be seen by the naked eye, but this same sensitivity can create night vision goggles (NVG) compatibility problems with cockpit displays. The compatibility issues fall into three categories. Category 1, 2 & 3 are, respectively, display emissions that are directly in the NVG's field of view, display emissions reflected into the NVG's field of view or display emissions diffusely scattered into the NVG's field of view. Category 1, 2 or 3 display emissions cause loss of contrast in the scene being viewed with the NVG. The contrast reduction leads to limited viewability and impaired object recognition, and it is known as NVG blooming or NVG flare.
The bloom effect is undesirable for two reasons. First, the bloom effect prevents the wearer from seeing the operational environment clearly and in fine detail. Second, the night vision goggles require a certain amount of time to be reset after a bloom effect event. Accordingly, the bloom effect is undesirable when operating a vehicle or aircraft in night vision conditions.
Conventional avionic displays designed to be utilized with NVIS equipment generally are restricted to a narrow emission, such as, single color (e.g., green) displays. The narrow emission is chosen so that it does not interfere with NVIS equipment. However, the restriction to the narrow emission significantly reduces the readability of information and the symbology provided on the displays. Further, it is difficult to highlight and differentiate large amounts of information on the display if the display is restricted to a single color.
Other conventional avionic systems have included color displays that include an NVIS filter. The color display operates in two modes: an NVIS mode (e.g., low luminance) and a daylight mode. The NVIS filter is provided between a light source used in the NVIS mode and an optical shutter, such as a liquid crystal display. The filter prevents emissions that cause NVIS equipment to bloom.
In the daylight mode, the displays use a second light source to provide light directly through the optical shutter without traversing the filter. The second light source is positioned so that its light is not provided through the NVIS filter.
Conventional NVIS filters are generally comprised of glass or other material supplemented by thin dielectric film coatings that attenuate infrared emissions or transmissions. Conventional NVIS filters typically use a thin film, multi-layer dielectric to obtain a sharp cutoff, with the knee starting between 600 nm to 630 nm. Additionally, they use an absorptive substrate to attenuate longer wavelength emissions. The conventional approach can produce a precise spectral cutoff for one viewing angle, but the cutoff shifts to shorter wavelengths with increasing viewing angle. (Reference: Optical Thin Films User's Handbook, James D. Rancourt, McGraw-Hill Optical and Electro-Optical Engineering Series, p. 68) This characteristic is particularly problematic because the wavelength at which NVIS goggles are sensitive is extremely close to the wavelength at which red emissions exist. Accordingly, a precise and stable cutoff frequency is needed in NVIS filters so that red colors can be effectively utilized on a display.
Certain conventional active matrix liquid crystal displays (AMLCDs) utilize two basic approaches for NVIS compliant backlighting. Both approaches have disadvantages associated with cost, space, and display quality.
In the first approach, a single lighting source comprised of a fluorescent lamp or light emitting diodes (LEDs) is utilized in combination with a large area infrared (IR) cutoff filter (a single mode AMLCD). The IR cutoff or NVIS filter is typically a thin film dielectric stack having a surface area equal to the surface area of the AMLCD. The NVIS filter can cost $1,000 or more and is disposed in the optical path. The conventional NVIS filter can cause undesirable display performance, such as reduced backlight efficiency, red de-saturation and reduced display luminance. The conventional thin film dielectric stack also can cause viewing angle performance issues.
In the second approach, at least two lighting sources are utilized to provide a daytime and nighttime operating mode, (a dual mode AMLCD). The daytime mode utilizes either a fluorescent lamp or an LED array, and the nighttime mode utilizes a wave-guide illuminated with fluorescent stick lamps or strips of LEDs. The illumination is directed through an IR filter and into the thin edges of the wave-guide. This approach has the advantage of removing the IR filter from the daytime optical path and greatly reduces the size of the IR filter. Although this approach provides a more efficient backlighting and a less expensive filter, the design of the wave-guide is complex and assembly of the AMLCD is more expensive and time consuming. Further, dual mode AMLCDs which use wave-guides require additional space around the perimeter of the AMLCD. The additional space is not available in certain space critical applications, such as on a 5ATI display or other avionic display. Also, this approach can suffer from light leaks leading to poor NVIS performance.
Further, future displays for military, sports, and transportation activities may utilize emitting technologies such as organic light-emitting diodes (OLEDs). One such technology involves flexible emissive displays. It is difficult to manufacture flexible, thin NVIS filters from conventional materials. Further, conventional NVIS filters such as thin film dielectric stacks are reflective in high ambient or daytime lighting, thereby reducing the contrast ratio of the display. A conventional thin film NVIS filter can reflect as much as 50% of the light that strikes it at the 630 nm wavelength.
Thus, there is a need for ambient lighting and display systems that can utilize inexpensive NVIS filters. Further, there is a need for a single mode display system which utilizes an inexpensive NVIS filter. Further still, there is a need for a system which can utilize an inexpensive filter having a relatively precise and stable cutoff frequency. Yet further still, there is a need for an avionic display which can utilize an inexpensive NVIS filter.
There is also a need for a dual mode display which does not require the complexity associated with wave-guides. Further, there is a need for an NVIS display which does not require additional space about the perimeter of the display. Even further, there is a need for an AMLCD display which can accommodate night vision equipment and yet is low cost, compact, and does not suffer from performance losses. Yet even further, there is a need for an NVIS filter for flexible emissive displays which is not as detrimental to contrast ratio in high ambients as conventional materials. Further still, there is also a need for a low cost filter material for absorbing infrared radiation which possesses a sharp spectral cutoff.